Super Optimal Broth (SOB) or SOB + Catabolite repression (SOC) are rich media used for the culturing of competent E. coli and recovery following transformation. SOB is usually used for the culturing and SOC for recovery.
Not that the "super optimality" is only in the context of making chemically-competent and electrocompetent cells due to its minimization of monovalent ions and saturation of cell membranes with divalent Mg²⁺.

Tryptone–20 g/LDissolve with stir bar.

Yeast extract–5 g/L
NaCl10 mM0.584 g/L
KCl2.5 mM0.186 g/L
Milli-Q water
Reserve Mg soln volumes
NaOH, 1 M
→ pH 7.0Check pH. typically not needed
Autoclave 15 min ≤ 1 L, 20 min 1–1.4 L.
Or filter sterilize* after adding components below.
MgCl2 sterile10 mM10 mL/L 1 Mautoclaved or filtered
MgSO4 sterile10 mM10 mL/L 1 Mautoclaved or filtered
Glucose sterile

20 mM
3.6 g/L

20 mL/L 1 M
7.2 mL/L 50%(500 g/L)
18 mL/L 20%(200g/L)

For SOC only.
Can be spiked-in.

Store at 4° for best long-term preservation of nutrients.
Aliquots of SOC can be frozen to prevent contaminant growth.

* Detergent-free membranes (PES, NYL, CN) are preferred for filter-sterilization.

  1. Dissolve the following with a stir bar:
    • 20 g/L tryptone
    • 5 g/L yeast extract
    • 0.584 g/L NaCl (10 mM)
    • 0.186 g/L KCl (2.5 mM)
    • sufficient Milli-Q water, subtracting Mg and glucose volumes below
    For highest competence of chemically-competent cells, use detergent-free glassware
  2. Measure the pH; it is usually already at or slightly above 7.0, so the traditional protocol instructing adjusting pH to 7.0 with NaOH is usually moot.
  3. Autoclave on the 15 min liquid cycle for ≤1 L, 20 min for 1–1.4 L.
    Or filter-sterilize after adding Mg solutions below, preferably with a detergent-free PES, NYL, or CN membrane.
  4. Add sterilized MgCl2 and MgSO4 to a final 10 mM each. For use as a recovery medium, you may choose just one of these, as 10 mM is sufficient Mg. The chloride salt is slightly superior.
    Omit glucose for SOB; add glucose for SOC.
    • 10 mM MgCl2: 10 mL/L 1 M sterile, or 5 mL/L 2 M
    • 10 mM MgSO4: 10 mL/L 1 M sterie, or 5 mL/L 2 M
    • 20 mM glucose: 20 mL/L 1 M, 18 mL/L 20%m/V (200 g/L), or 7.2 mL/L of 50%m/V (500 g/L)
      For SOC only; glucose can be spiked into SOB when needed, without regards for its volume.
  5. Store at 4° for best long-term preservation of nutrients. Aliquots of SOC can be frozen to inhibit contaminant growth, unlike storage at 4° which can still allow contaminant growth.


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